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Molecular Orbitals

Molecular Orbitals. Chapter 9. Molecular Orbital model. This model examines unpaired electrons, bond energies and excited state electrons. Examine the H 2 molecule. When 2 atomic orbitals overlap, two molecular orbitals form.

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Molecular Orbitals

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  1. Molecular Orbitals Chapter 9

  2. Molecular Orbital model • This model examines unpaired electrons, bond energies and excited state electrons. • Examine the H2 molecule. • When 2 atomic orbitals overlap, two molecular orbitals form. • One low energy bonding orbital and one high energy anti-bonding orbital

  3. Electron Probability Distribution • The electron probability of both MO and MO* is centered along the line passing between the nuclei. • In the MO, the greatest e- probability is between the nuclei. • In MO* the greatest e- probability is on either side of the nuclei

  4. Describing the bonding • The molecular orbitals are constructed from the atomic 1s orbitals of the H atoms. • The resulting molecular orbital may be represented as : • MO1 = 1sa +1sb • MO* = 1sa – 1sb

  5. Important points • The e- probability is centered along the line passing between the two nuclei.. • This type of electron distribution is described as sigma (σ). MO1 & MO* are sigma molecular orbitals. • In this model only molecular orbitals are available for occupation by electrons. Atomic orbitals no longer exist because the molecule , a new entity, has its own set of new orbitals.

  6. Bonding & Anti-bonding

  7. More points • MO1 is lower in energy than 1s of the free H atom. • This is the driving force behind molecular formation, and stability. • If the electrons are forced to occupy the higher energy MO*, or anti-bonding orbitals, that reduces the stability of the molecule. • Bonding MO’s are lower energy than individual atomic orbitals. • Anti-bonding MO’s are higher energy than individual atomic orbitals.

  8. Last points • Molecular orbitals may be written like electron configurations. σ 1s2 etc. • Each molecular orbitals holds up to 2 electrons with opposite spin. • Orbitals are conserved. The number of molecular orbitals will always be the same as the number of atomic orbitals used to construct them.

  9. Examples

  10. Bond Order • Bond order is defined as bond order = ½ [(# of bonding e-) - (# of anti-bonding e-)]

  11. More examples • Oxygen has a bond order of 2, but it also shows two unpaired electrons. • ½(10-6)=2 • The larger the bond order, the greater the bond strength.

  12. Still other examples

  13. Try it • Which is stable, He2 or Li2? • With a bond order of 0 versus 1, Lithium is more stable. • Which is more stable, O2, O2+ or O2- ? • 2, 2.5 and 1.5 indicate that O2+ is the most stable.

  14. Paramagnetism • Unpaired electrons exhibit paramagnetism. • In this picture liquid oxygen is held in a magnetic field until it boils away.

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